1. Field of the Invention
The present invention generally relates to implantable medical devices, and more specifically, relates to sense amplifiers for implantable pulse generators.
2. Description of the Prior Art
Modern implantable pulse generators used in cardiac pacing commonly employ the demand or inhibited mode. In this technique the implantable pulse generator provides a pacing pulse only when a corresponding naturally occurring pacing pulse is not sensed within an appropriate period of time, called the escape interval. This sensing is normally accomplished by timesharing the wire or lead which conducts the pacing pulse from the implantable pulse generator to the cardiac tissue to be stimulated. The sensing activity is turned off whenever the lead is used to deliver a pacing pulse.
Many present day cardiac pacing implantable pulse generators both pace and sense in two chambers of the heart. That is both atrial and ventricular chambers are sensed for naturally occurring pacing pulses, and if not detected within the predetermined escape interval, an artificial pacing pulse is delivered to that chamber. Because of the characteristics of the artificial pacing pulses and the passband of the sense amplifiers, it is common to blank a sense amplifier during delivery of an artificial pacing pulse to another chamber. This is a particularly critical feature for the ventricular sense amplifier because of the relatively short interval between atrial and ventricular stimulation. This period of blanking is present on most modern dual chamber pulse generators.
The earliest demand pacers utilized analog sense amplifiers. These were implemented with discrete circuitry. As monolithic device technology became more available, attempts were made to manufacture monolithic sense amplifiers. The major difficulty was the need to use discrete resistors to implement R/C filters for the necessary bandpass filtering. The use of such discrete resistors added to the cost and complexity of the manufacturing processes.
A major breakthrough was made by Beck as described in U.S. Pat. No. 4,649,931. Beck applied switched capacitor circuitry to provide bandpass filtering in a pacing sense amplifier. This technique eliminates the use of resistors in the filtering circuit, relying instead on measuring the charge digitally switched into and out of an integrating capacitor.
A problem in bandpass filtering peculiar to the switched capacitor approach occurs if transients are present at a rate which interferes with the switching rate. This is not a problem when blanking a sense amplifier to time share the lead because the logic which generates a stimulation pulse can also compensate for these transients. Also the circuitry has almost an entire cardiac cycle to purge the transient from the system before sensing needs to begin.
The transient problem is most acute in the ventricular sense amplifier as a result of the period immediately following an artificial atrial pacing pulse. The difficulty is exacerbated by the relatively short time interval between atrial and ventricular contraction.